Boa: an Embedded Web Server

Most of the world thinks of a web server
as something on the Internet that can help find a football score,
access a bank account or find an airplane flight. That's well and
good but, in the embedded world, there are lots of places where a
web server could come in handy. A common example is a
network-connected printer. Being able to inquire about the status
of the printer is useful, and if a small enough web server is
available, it could offer a convenient interface.

This isn't just theory. Tektronics, for example, includes a
web server in its 780 color laser printer. Their server allows you
to configure the printer, access documentation, inquire about
printer status--including toner levels--and includes links for
ordering supplies. I am sure the cost of the web server is offset
by supplies revenue and decreased support costs.

The most popular web server for traditional web sites is
Apache. Apache is full-featured, very configurable and very
reliable. It has proven itself in the general-purpose web server
market and has been adopted by major players, including IBM.

Apache's design requires it to spawn (fork and exec) a
separate task for each simultaneous connection. This is not
necessarily a bad design, but in the embedded world, it is likely
that you will need something that uses fewer resources--an average
memory footprint in the tens of thousands of bytes rather than in
the megabytes, for example.

Enter Boa

Boa is a tiny web server that also offers extremely high
performance. It is specifically designed to run on UNIX-like
systems, which includes Linux, as well as the *BSD systems. To get
all the legal stuff out of the way, Boa is available for free and
is covered by the GNU Public License (GPL). The source code for
Boa, as well as documentation, can be found at
http://www.boa.org/.

The following point taken from the Boa web site shows why I
have decided to write about Boa:

Boa currently seems to be the favorite web server
in the embedded crowd, and embedded Linux, despite all the
marketing hype, really is a big deal. Supposedly, an older version
of Boa, v0.92q, runs in 32K address space on m68k, like used in
uClinux. See
http://www.uclinux.net/.

Unlike traditional web servers, Boa is single-tasking. What
this means is that it internally multiplexes all of the ongoing
HTTP connections rather than the more traditional forking of
multiple copies for each connection. It must, of course, fork
separate processes to execute CGI scripts, but that is well defined
and under the control of the system designer.

My use of Boa has been to handle very low-volume HTTP
connections on the order of a few hundred to a few thousand pages
served per day. The Boa web page, however, claims that tests show
Boa capable of handling several thousand hits per second on a
300MHz Pentium and dozens of hits per second on a 20MHz 386/SX.
Thus, for most applications, it's unlikely that performance is
going to be an issue.

Of course, in an embedded application, it is likely that the
primary job of the processor will be handling something other than
running a web server. What this means is that running Boa is not
likely to significantly decrease the resources available for the
primary purpose of the embedded CPU.

This small size and high performance doesn't come for free.
However, what Boa doesn't do generally woudn't be missed for an
embedded system. Here is the short list:

The REMOTE_HOSTNAME environment variable is not
set. What this means is that a CGI program doesn't have the name of
the accessing host available. The REMOTE_ADDR variable is
available, so if the CGI program actually cares, it can do its own
lookup.

Boa does not support server-side includes. SSI is
slow because the text of each requested document must be searched
for each request.

The ability to change root directories (chroot) is
not available.

Rather than offering access control features, Boa
just uses Linux/UNIX filesystem permissions to determine what can
be served. This is generally not a problem as Boa can be configured
to run as any user ID you desire.

Setting up Boa

Because Boa is so compact, it is also very easy to install
and configure. The details are on the Boa web site, but if you are
familiar with installing typical tarballs, Boa will be no surprise.
The basic steps are:

Download the software.

Unpack using tar xvzf.

Optionally edit the defines.h file in the src
directory to change the default SERVER_ROOT. This isn't mandatory;
you can also specify the server root on the command line when you
start Boa.

Run the configure script
(./configure) and then type
make to build the executable. Recently, I built
Boa on my laptop. The make step took less than 11 seconds.

Edit the configuration file, boa.conf, to set up
your local configuration.

Make sure the appropriate directories (such as the
log directory) exist and have the proper permissions.

Start Boa from the command line. Use the -c
command-line option to specify the server root if you didn't set
SERVER_ROOT to the desired directory location in define.h.

That's it. You should have a working version of Boa. Point
your web browser at it and see what happens. If it doesn't work,
check the error_log file.

Comments

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This would be great site for me.I really admire you Sid because you admit your mistakes and also you offer us good choice.Your 20 years of experiencing working in computer is a good advantage to you.Good luck and more power.....

This would be great site for me.I really admire you Sid because you admit your mistakes and also you offer us good choice.Your 20 years of experiencing working in computer is a good advantage to you.Good luck and more power.....

Boa is a really good webserver but it doesn't offers all the options and the modules of an apache webserver, so who says apache can't be embedded?, I have done it with, foXServe a software platform for fox LX 832 SBC. I have also ported mod_ssl for cryptographt, mod_dav for mounting it as a web drive and PHP for pre-processing so a full mini server in just 66mmx72mm and with a lot more then BOA!!! Apache 1.3.x if right configured is not bigger than BOA. Consider also that Apache architecture is more scalable than BOA and can support more connections if fine tuned!